The invention relates to an extrusion head for producing a plastic strand in the form of a flexible or rigid tube. The invention also relates to an extrusion blow molding machine equipped with an extrusion head.
The usual containers of the past, made of tin plate, nonferrous sheet metal, glass, or even ceramic, are increasingly being replaced with containers of plastic. Particularly for packaging fluid substances such as beverages or oil, cleaning utensils, cosmetics, and so forth, plastic containers are used primarily. In this substitution, the low weight and lower costs surely play a not inconsiderable role. The use of recyclable plastic materials, which can be produced with more-favorable overall energy consumption, also helps promote consumer acceptance of plastic containers, and especially plastic bottles.
Plastic containers and especially plastic bottles, for instance of polyethylene or polypropylene, are produced by the extrusion blow molding process, and especially a process for blow molding flexible tubes. First, with the aid of an extrusion head, a one- or multi-layer flexible plastic tube in the form of a flexible or rigid tube is extruded, for example, from thermoplastic. A continuously extruded flexible plastic tube-is placed in a blow molding tool, cut off, blown over a blow molding mandrel by overpressure, and cured by cooling. The extrusion blow molding machines used for this purpose as a rule have at least one extruder for delivering the plastic material to the extrusion head. The outlet of the extruder communicates with the extrusion head, which has at least one extrusion nozzle the opening width of which can be regulated. For the sake of continuous operation, in the known extrusion blow molding machines two stations with blow molds are provided, which are served in alternation by the extrusion head. The blow molding stations are disposed opposite one another on both sides of the extruder and have blow molding tables that are moved to beneath the extrusion head in alternation in order to receive the extruded flexible tube. Overall, the extruder with the extrusion head and the two blow molding stations is approximately T-shaped. The extruder and the extrusion head form the long bar of the T, while the two blow molding tables are movable in alternation along the short halves of the crossbar to beneath the extrusion head.
The extrusion head communicates with the outlet of the extruder and is supplied by it with, for example, the thermoplastic material. At least one extrusion nozzle is provided on the extrusion head. Often, the extrusion head is equipped with a plurality of extrusion nozzles, so that in a single operation, a plurality of flexible plastic tubes can be produced simultaneously and taken over by a multiple blow molding tool. Each extrusion nozzle has a nozzle tool, which has a central mandrel and a nozzle ring surrounding the mandrel. The annular gap defined by the jacket of the mandrel and the inside surface of the nozzle ring forms the outlet opening of the extrusion nozzle. The mandrel typically has a shape that widens or narrows conically toward its free end. The inside face of the nozzle ring is typically also a conical jacket face. The angle of inclination of the inside face of the nozzle ring relative to the mandrel axis differs from the angle of inclination of the jacket face of the mandrel. By means of an axial relative motion between the mandrel and the nozzle ring, the opening width of the outlet opening can be varied, and as a result the wall thickness of the extruded flexible plastic tube can be controlled. The relative motion is usually effected by means of raising and lowering the mandrel relative to the immovably disposed nozzle ring. In the prior art, embodiments are also known in which the nozzle ring is axially movable, while the mandrel is fixed.
For the axial relative motion between the mandrel and the nozzle ring, a hydraulic or electric actuating drive is provided, by way of which the relative motion can be performed in accordance with predetermined criteria. The actuating drive cooperates with the mandrel or the nozzle ring to regulate the opening width of the outlet opening and as a result to modify the wall thickness of the extruded flexible plastic tube as specified. Given the mass that has to be moved, particularly upon an adjustment in height of the nozzle ring, and in view of the speed and precision of the adjustment, the motors and gears used for the actuating drive are relatively complicated and expensive. It is often necessary to change the extrusion head, for instance in order to adapt the extrusion blow molding machine to different plastics, or to change from a single-layer extrusion head to a multi-layer extrusion head, or to adapt the extrusion head to different shapes of container, different blow molding tools, and the like. The extrusion heads known from the prior art are embodied as a structural unit, which can be replaced only in its entirety. In the known extrusion blow molding machines, it is therefore necessary for the entire extrusion head, with the nozzle tool and actuating drive, to be unflanged from the extruder and replaced with a new extrusion head. The newly installed extrusion head must then be adapted to the altered conditions and recalibrated in its entirety. Taking the enormous weight of the extrusion head into account, it is immediately apparent that replacing and calibrating the entire extrusion head is a very time-consuming and labor-intensive process. The actuating drive for the nozzle tool is a fixed component of the tubing heads known from the prior art and is not a negligible expense item involved in changing the extrusion head. Moreover, when the known tubing heads are changed, the hydraulic and/or electrical installations for the actuating drive must be redone, to assure reliable operation of the thus altered extrusion blow molding machine.